This invention relates to a plasma display panel material for use in a plasma display panel.
Generally, the plasma display panel comprises a front glass plate and a rear glass plate arranged opposite to each other with a predetermined gap kept therebetween. The front glass plate is provided with a plurality of discharging electrodes formed thereon to perform a plasma discharging operation. A dielectric layer having a thickness of about 30 to 40 xcexcm is formed on the front glass plate to cover the discharging electrodes. The dielectric layer serves to maintain the plasma discharging operation and to protect the discharging electrodes and is required to have a high withstand voltage or breakdown voltage and an excellent transparency.
Similarly, the rear glass plate is provided with a plurality of discharging electrodes for performing a plasma discharging operation and a dielectric layer for maintaining the plasma discharging operation and for protecting the discharging electrodes. Furthermore, a plurality of barrier ribs are formed to define on the discharging electrodes and the dielectric layer a number of gas discharging sections known in the art.
As a method of forming the above-mentioned dielectric layer, use has been made of a technique including the steps of preparing a paste of a dielectric material comprising glass powder or a mixture of glass powder and ceramics powder, applying the paste by printing or batch coating onto the glass plate with the discharging electrodes formed thereon, and firing or baking the paste, a technique including the steps of forming the dielectric material into a green sheet, adhering the green sheet onto the glass plate with the discharging electrodes formed thereon, and firing the green sheet, and so on.
With any existing dielectric material, it is difficult to form a coating film which has a flat surface and a uniform thickness and which includes less bubbles remaining in the film. Therefore, the dielectric layer formed as the coating film after firing often suffers presence of a number of bubbles remaining therein and occurrence of cracks formed therein. In this event, the dielectric layer is not transparent but opaque and dielectric breakdown may be caused to occur. Thus, when the plasma display panel having such dielectric layer is produced, an image formed thereon is not clear but is uneasy to watch. Furthermore, the dielectric layer is often colored yellow as a result of reaction between the dielectric layer and Ag electrodes as the discharging electrodes formed adjacent to the dielectric layer. In this event, the image formed on the plasma display panel is difficult to watch.
In order to obtain the dielectric layer having a flat surface and a uniform thickness, Japanese Unexamined Patent Publication (JP-A) No. H11-209147 proposes a dielectric paste having a moisture content adjusted to 3 wt % or less by controlling a humidity in a paste preparing step. However, this paste does not address to the reaction between the dielectric layer and the Ag electrodes at all.
It is therefore an object of this invention to provide a plasma display panel material capable of forming a dielectric layer which has a flat surface, a uniform thickness, and an excellent surface condition with less bubbles remaining in the dielectric layer and capable of preventing the dielectric layer from being colored yellow as a result of reaction between the dielectric layer and Ag electrodes.
As a result of extensive studies, the present inventors have found out that water or moisture present on the surface of glass powder or ceramics powder affects the surface condition of a coating film formed as the dielectric layer or the occurrence of yellowish coloring of the dielectric layer by Ag electrodes and that the above-mentioned objects are achieved by controlling the moisture content. Based on the finding, this invention has been made.
According to this invention, there is provided inorganic powder as a plasma display panel material, wherein the inorganic powder comprises a powdery material containing glass powder, the powdery material having a moisture content within a range between 0.1 and 2 mass %.
According to this invention, there is also provided a paste as a plasma display panel material, wherein the paste contains glass powder as an inorganic component, the inorganic component having a moisture content within a range between 0.1 and 2 mass %.
According to this invention, there is provided a green sheet as a plasma display panel material, wherein the green sheet contains glass powder as an inorganic component, the inorganic component having a moisture content within a range between 0.1 and 2 mass %.
In the above-mentioned plasma display panel material such as the inorganic powder, the paste, and the green sheet, the moisture content in the glass powder or a mixture of the glass powder and ceramics powder is adjusted to 2 mass % or less. Therefore, no adverse influence is given to a viscosity characteristic of the paste or a sheet slurry prepared upon forming the green sheet so as to obtain as a dielectric layer a coating film which has a flat surface and a uniform thickness and contains less bubbles remaining in the film. Since the moisture content is adjusted to 0.1 mass % or more (preferably, 0.2 mass % or more), the reaction between the dielectric layer and Ag electrodes can be suppressed.
The mechanism how the above-mentioned reaction is suppressed by the moisture present on the surface of the powder is not clear but is supposed as follows. If no moisture is adsorbed to the surface of the powder, a resin contained in the paste or the sheet slurry is strongly adsorbed to the surface of the powder. In this event, the resin remains unremoved even after completion of a debinding process removing a binder known in the art. This provides a reducing atmosphere in a firing step so that Ag diffused into the dielectric layer from the Ag electrodes is reduced into a colloidal phase to cause yellowish coloring. However, if a predetermined amount of moisture is adsorbed to the surface of the powder as proposed in this invention, the debinding process is promoted so that yellowish coloring of the Ag is hardly caused to occur upon firing. In case where the moisture is present not on the surface of the powder but only in the paste or the sheet slurry, most of the moisture is lost in a drying process of the paste or the green sheet. As a result, the resin is strongly adsorbed to the surface of the powder so that yellowish coloring is easily caused to occur.
In this invention, the moisture content of the glass powder or the mixture of the glass powder and the ceramics powder is measured by the Karl-Fischer tiltration. The Karl-Fischer filtration is a method of measuring the amount of moisture by the use of the Karl-Fischer reagent comprising iodine, sulfur dioxide, pyridine, and methanol and having a nature of quantitatively or stoichiometrically reacting with the moisture. Herein, the amount of moisture released upon heating the powder is measured.
The moisture content may be adjusted by strictly and precisely controlling the moisture during a production process of the glass powder or the ceramics powder. In particular, it is important to control the amount of water added as a promoter upon pulverization of the glass. It is desired to add 0.1 to 2.5 mass % (preferably, 0.2 to 2 mass %) of water is added with respect to the amount of the glass to be pulverized.